热电离质谱(TIMS)测定Ca同位素时Sr干扰影响的实验评价

摘要

Calcium and strontium are all alkaline earth metals.Because of their similar chemical properties,it is difficult to separate strontium from calcium completely during chemical separation processes.Thus,the calcium cut after chemistry usually contains some strontium.Besides,it is known that Sr double charges,such as 88Sr2+,86Sr2+,84Sr2+,could be treated as interferences to 44Ca+,43Ca+,42Ca+ respectively,during calcium isotope measurements on thermal ionization mass spectrometry (TIMS).To evaluate this interference effect,we designed an experiment and ran a series of samples on TIMS using single Ta filaments.IAPSO seawater standard was used in the experiment,and a certain amount of Sr standard solution was added in it to make the mixture has a Ca/Sr ratio about 5.The mixture was then divided into several equal solutions and pass through the calcium columns respectively.For each column,different percentage of Sr cut was collected together with whole Ca cut and calcium ratio was determined thereafter on TIMS using single Ta filaments.As the TIMS measurements' result,all of those samples,although having different Ca/Sr ratios,display unique calcium isotopic compositions with IAPSO seawater standards that our laboratory monitoring for a long time within error.Based on the results,we conclude that currently,for the most geology samples,that usually contain a Ca/Sr ratio＞5,Sr interference effect on Ca measurement on TIMS using single Ta filament is very limited,which could be neglected.Besides,the chemical separation procedure for Ca could be improved,through increasing the concentration of the leaching acid,better Ca peak shape and efficiency can be reached.%Ca和Sr属于碱土金属元素,具有相似的化学性质,在Ca的纯化富集过程中很难实现Ca和Sr的完全分离,接取Ca的淋洗液中不可避免地会混入少量Sr.所以,在使用热电离质谱(TIMS)进行Ca同位素测定时,88Sr2+、86 Sr2+、84Sr2+可能会分别对44Ca+、43 Ca+、42Ca+的准确测定产生干扰.本工作通过设计实验来评估Sr使用Ta单带在TIMS测定Ca同位素过程中的影响:以IAPSO大西洋标准海水为研究样品,加入Sr元素单标溶液,使得样品中Ca/Sr=5;通过接取不同体积的Sr元素淋洗液,以获得含有不同Ca/Sr比值的TIMS测试样品.实验结果表明,含有不同Ca/Sr比值的测试样品与不含Sr元素的海水样品的Ca同位素组成在分析误差范围内一致.由此可以认为,对地质样品(Ca/Sr＞5)用Ta单带在TIMS上进行Ca同位素分析测试时,在当前技术条件下,Sr对Ca同位素的影响有限,可以忽略.另外,在进行Ca化学分离时,可以适当增加淋洗酸的浓度,以获取好的峰型并提高效率.